It is well known to prepare photosensitive compositions comprising a photosensitive agent and a polymeric binder and to utilize such compositions in the preparation of lithographic printing plates which can be developed with aqueous neutral or alkaline developing solutions. In such compositions, it is common practice to utilize a diazo resin as the photosensitive agent. The lithographic printing plates typically comprise a metal substrate, such as a substrate of anodized aluminum, which is coated with a photo-sensitive composition comprising the diazo resin, the polymeric binder and other ingredients such as colorants, stabilizers, exposure indicators, surfactants, and the like.
Many different polymers have been proposed for use as binders in the aforesaid photosensitive compositions. Particular advantages have been achieved by the use of acetal polymers as described, for example, in U.S. Pat. Nos. 4,652,604, 4,741,985, and 4,940,646.
In U.S. Pat. No. 4,652,604, the acetal polymer contains acetal groups of three types, namely six-membered cyclic acetals, five-membered cyclic acetals and intermolecular acetals. In U.S. Pat. No. 4,741,985, the acetal polymer is a mono-acetal containing a six-membered cyclic acetal group. In U.S. Pat. No. 4,940,646, the acetal polymer contains vinyl acetal units derived from an aldehyde that contains hydroxyl groups.
Acetal polymers can be advantageously utilized in lithographic printing plates in which the layer containing the acetal polymer and the diazo resin is the sole radiation-sensitive layer. They can also be usefully employed in so-called "dual layer" plates. In this type of lithographic printing plate, a radiation-sensitive layer containing a diazo resin is coated over an anodized aluminum support and a radiation-sensitive layer containing a photocross-linkable polymer is coated over the layer containing the diazo resin. Such dual layer plates are described, for example, in British Patent No. 1 274 017. They are advantageous in that radiation-sensitive layers containing diazo resins adhere much more strongly to most anodized aluminum supports than do radiation-sensitive layers containing photocrosslinkable polymers. Thus, the enhanced performance provided by photocrosslinkable polymers is achieved without sacrificing the excellent adhesive properties of diazo resin compositions.
Copending commonly assigned U.S. patent application Ser. No. 738,068 filed Jul. 30, 1991, "Binary Acetal Polymers Useful In Photosensitive Compositions And Lithographic Printing Plates" by John E. Walls describes an important advance in the art relating to the use of acetal polymers in radiation-sensitive compositions and lithographic printing plates. The novel acetal polymers described in this patent application are binary acetal polymers exhibiting improved properties in comparison with previously known acetal polymers. They provide lithographic printing plates characterized by improved abrasion-resistance, improved resistance to chemical attack, extended press performance and enhanced rollup properties. The binary acetal polymers can be trimers, in which the recurring unit comprises a vinyl acetate moiety and first and second cyclic acetal groups, or tetramers in which the recurring unit comprises a vinyl alcohol moiety, a vinyl acetate moiety and first and second cyclic acetal groups. Both of the acetal groups are six-membered cyclic acetal groups, one of them is unsubstituted or substituted with an alkyl or hydroxyalkyl group and the other is substituted with an aromatic or heterocyclic moiety.
More specifically, the binary acetal polymers described in the aforesaid copending patent application are polymers with recurring units represented by the formula: ##STR1## R.sub.5 =--OH, --CH.sub.2 OH, --OCH.sub.3, --COOH or --SO.sub.3 H z=1 to 3
and
n.sub.1 =0-25 mole %, preferably 5 to 15 mole % PA1 n.sub.2 =2-25 mole %, preferably 5 to 15 mole % PA1 n.sub.3 =10-60 mole %, preferably 15 to 35 mole % PA1 n.sub.4 =35-80 mole %, preferably 45 to 70 mole %. PA1 n.sub.2 =2-25 mole %, preferably 5 to 15 mole % PA1 n.sub.3 =10-70 mole %, preferably 15 to 50 mole % PA1 n.sub.4 =10-60 mole %, preferably 12 to 45 mole % PA1 n.sub.5 =10-45 mole %, preferably 15 to 30 mole %.
While the novel binary acetal polymers described above provide many important advantages, they have the disadvantage that they will not fully dissolve in aqueous developing solutions, but rather break-up in the form of particles which tend to re-deposit on the imaged areas of the printing plate. As a consequence of the particulate dissolution, there tends to be an undesirable build-up of residue in the processing equipment, such as on the brushes and/or the plush. A polymeric binder that dissolves fully in the developer solution affords better and surer cleanout of shadow screen areas, avoids coating residue build-up in the processing equipment, and minimizes the need to change filters.
It is toward the objective of providing improved photosensitive compositions and improved lithographic printing plates, capable of effectively meeting the needs of the lithographic printing plate art, that the present invention is directed.